Now that I think, maybe I did Alt-Tab, since it does not happen everytime, but usually I do Alt-Tab when I am waiting for a game. I will try the Alt-Enter and see if it helps.
Sometimes it happens that I can't move my screen with my mouse down and right. I have to resort to moving with cursor keys or clicking on the map, which is really annoying, since I am used to moving screen with mouse drag.
Reseting the game fixes it, but usually it happens in the middle of a game.